The present invention is directed to a weigh feeder. In particular, the invention is directed to a weigh feeder which comprises a digital weight transmitter operatively associated with a conveyor belt for providing plural frequency signals which are indicative of the weight of material on the belt.
Digital weigh feeders are well known. For example, see U.S. Pat. No. 3,724,720 entitled "Digital Mass Flow Control System" assigned to the assignee herein. These weigh feeders are termed "true rate" systems since they control the true or instantaneous rate of the mass flow of material. In such systems, belt speed is automatically adjusted as a function of the weight of material sensed on the belt to achieve a constant feed or mass flow rate. Specifically, belt speed and belt loading are sensed and multiplied together to obtain an actual mass flow feedback signal. This feedback signal is compared to a desired flow rate or set point signal. Any difference between the two signals causes a change in belt speed to achieve a steady flow of material.
The heart of any weigh feeder is the weight sensing component. The performance demands on such a component may be awesome. The ideal weight sensing component must be highly precise, exactly repeatable, completely linear and capable of operating over a broad range of loading conditions while remaining rugged and stable through long use with little or no calibration. The ideal weight sensing component would require no stabilization time, would not deflect under loading, and would be immune to vibration and temperature extremes. Heretofore, the weight sensing components used in weigh feeders fell far short of the ideal in respect to many of the above requirements. As such, the weigh sensing components considerably limited the performance of weigh feeder systems.
A vibrating string digital weight transmitter closely approximates the above criteria for an ideal weight sensing component. Such a digital weight transmitter is described in U.S. Pat. Nos. 3,411,347, 3,423,999, 3,621,713, 3,763,971, 3,724,573 and 3,805,605. The transmitter offers unexcelled accuracy and stability. The transmitter senses a load digitally so that loading information is not subject to inaccuracies due to drift which inaccuracies are common in analog weight sensors. The vibrating string transmitter is extremely accurate and may provide repeatability of .+-.0.003%, linearity of .+-.0.03% and long term stability of .+-.0.03% over considerable periods of time. The vibrating string transmitter closely approximates a defectionless load sensing system and is virtually immune to temperature and vibration extremes which typify the weigh feeder environment.
The vibrating string digital weight transmitter provides plural output signals at different frequencies. The ratio of the frequencies provides an indication of the weight being sensed by the transmitter. Weigh feeder systems known in the art are not compatible with such weight transmitters. Accordingly, such weigh feeders cannot secure the benefits in accuracy, stability and reliability of a vibrating string transmitter.
An advantage of the present invention is that it is totally compatible with a vibrating string digital weight transmitter.
Another disadvantage of the invention is that it is much more accurate, stable and reliable than weigh feeders known in the art.
A further advantage of the invention is that it automatically computes the average tare weight of a conveyor belt and provides an accurate indication of the actual weight of material on the belt.
Other advantages appear hereinafter.